1. Field of the Invention
The present invention relates to a method for manufacturing a high pressure discharge lamp having a high luminous flux maintenance factor and a long life, a high pressure discharge lamp manufactured using this method, a lamp unit, and an image display device.
2. Description of the Related Art
In recent years, projection-type image display devices such as a liquid crystal projector and a DMD (Digital Micromirror Device) projector are widely used as systems that realize large-screen images. High pressure discharge lamps having high luminance, especially high pressure mercury lamps, are often employed as light sources of such image display devices (see Japanese Patent Application Publication No. H02-148561 as one example).
FIG. 1 shows a construction of a high pressure mercury lamp 1000 disclosed by the above publication.
In the drawing, the high pressure mercury lamp 1000 has a light emitting part 501 which is mainly made of quartz, and one pair of sealing parts 502 extending from both sides of the light emitting part 501. A metal electrode structure is sealed in each of the sealing parts 502, to make the inside of the light emitting part 501 airtight while allowing power to be supplied from outside into the light emitting part 501.
The electrode structure is formed by electrically connecting an electrode 503 made of tungsten (W), amolybdenum (Mo) foil sheet 504, and an external lead 505 in this order. A coil 512 is wound around a tip of the electrode 503.
Mercury (Hg), which is a light emitting material, argon (Ar), and a small amount of halogen gas are enclosed inside the light emitting part 501.
When a starting voltage is applied to the ends of the pair of external leads 505 of this high pressure mercury lamp 1000, a discharge of Ar occurs and the temperature in the light emitting part 501 increases. As a result of this temperature increase, Hg atoms evaporate and occupy the inside of the light emitting part 501 in gaseous form. During this time, though an Hg vapor pressure reaches as high as 15 MPa to 20 MPa, the airtightness can be maintained by the molybdenum foil sheets 504 in the sealing parts 502 (foil sealing structure).
There is a growing tendency to increase a charged pressure of mercury in such a constructed high-pressure mercury lamp 1000, in order to achieve a longer life and higher luminance.
However, when the charged pressure of mercury is increased, the molybdenum foil and the quartz glass in the sealing part 502 peel away from each other as over time, due to factors such as a difference in thermal expansion coefficient between the two materials. This causes a leakage of the substances enclosed in the light emitting part 501.
To solve this problem, Japanese Patent Application Publication No. 2002-93361, as one example, discloses a construction in which sealing is performed with an additional member, formed by adding a raw material such as copper oxide (CuO) or aluminum oxide (Al2O3) to silica (SiO2), being interposed between a portion of an electrode rod of the electrode located in the sealing part and the quartz glass which forms the sealing part. This produces greater adhesiveness between the sealing part and the electrode structure in an area where the additional member is provided. As a result, the molybdenum foil and the quartz glass do not peel away from each other, and leakage is thereby prevented.
Also, Japanese Patent Application Publications Nos. 2000-182566 and 2000-195468, for example, disclose high pressure mercury lamps in which the electrode structure is sealed in the sealing part through a functionally gradient material being interposed therebetween, thereby being able to withstand greater pressures.
FIG. 2 is a partial cutaway view showing a construction of a high pressure mercury lamp disclosed in Japanese Patent Application Publication No. 2000-182566. As illustrated, a block member 523 made of a functionally gradient material is fixed in each of two side tube parts 522 that extend from both sides of an arc tube 521 made of quartz glass, and a feeder 524 is sealed near an outer end of this block member 523.
The functionally gradient material referred to here is a material that has different thermal expansion coefficients in different portions. In the example of FIG. 2, the thermal expansion coefficient of the block member 523 is closer to that of quartz glass in a portion nearer the side tube part 522, and closer to that of a metal which forms the feeder 524 in a portion nearer the outside. In more detail, the block member 523 contains molybdenum as a conductive ingredient and silica as a nonconductive ingredient. One end of the block member 523 opposite to the arc tube 521 is rich with molybdenum and therefore conductive. Silica content increases in a continuous or stepwise manner in a direction toward the arc tube 521, such that the end of the block member 523 nearest the arc tube 521 is rich with silica and therefore nonconductive.
Such a block member 523 reduces the thermal stress which occurs in the contact area between different materials in the sealing part due to the difference in the thermal expansion coefficients of the different materials, to thereby suppress cracking and the like. In this way, the pressure resistance strength in the sealing part is enhanced.
Both of the above constructions, i.e. the sealing of the electrode structure via the additional member containing copper oxide or the like and the sealing of the electrode structure via the functionally gradient material member, certainly improve the pressure resistance strength in the sealing part and contribute to higher luminance of the high pressure mercury lamp. According to these constructions, however, blackening and devitrification tend to occur in the light emitting part during lighting, which shortens the service life of the high pressure mercury lamp.
This problem can be attributed to the following. Both the additional member containing copper oxide or the like and the functionally gradient material member inevitably contain impurities by their nature. When manufacturing or lighting the high pressure mercury lamp, such impurities unavoidably enter into a discharge space inside the light emitting part.
The impurities which have entered into the discharge space may react with quartz glass forming the inner wall of the light emitting part, especially in a high temperature area. This leads to devitrification. Also, the impurities, and in particular an alkali metal, may ionize and bind to a halogen which is enclosed in the discharge space. As a result, a halogen cycle cannot work properly, and tungsten evaporating from the electrode deposits itself on the inner wall of the light emitting part. This leads to blackening.
Efforts have been made to prevent impurities which are contained in the sealing part from entering into the light emitting part in the high-pressure mercury lamp, but no decisive solution has yet been proposed. This problem can occur not only in the high pressure mercury lamps but also in high pressure discharge lamps having sealing parts in general.
The present invention was conceived to solve the above problem, and aims to provide a method for manufacturing a high-pressure discharge lamp in which a functionally gradient material or an additional material, e.g. quartz glass with an additive, is disposed in a sealing part to increase a pressure resistance strength, such that the occurrence of blackening and devitrification in a light emitting part can be suppressed by removing impurities from a discharge space in the light emitting part in a simple manner. The present invention also aims to provide a high pressure discharge lamp manufactured using this method, a lamp unit, and an image display device.